A. Field of the Invention
The present invention relates to digital communication systems, and more particularly relates to on echo canceller touch-up period in an echo-cancelling modem.
B. Description of the Related Art
Although arbitrarily high transmission rates can be attained over special-purpose, wideband transmission circuits, such links are typically unavailable and transmission must accordingly be handled by conventional, voice-grade, dial-up telephone lines. Such standard phone lines, however, have severely limited bandwidth, are subject to line noise, and typically exhibit line irregularities and poor termination, making them echo-prone. These factors severely complicate the task of improving modem transmission speeds over dial-up lines. Better use of the roughly 3,400 KHZ bandwidth available on the dial-up network has been realized by using advanced modulation and equalization techniques. Modems capable of operating at high speeds, such as those conforming to the CCITT V.32 Bis standard, achieve this by using a modulation method called "quadrature amplitude modulation" (QAM), in which both the amplitude and phase of the signal are modulated.
Echo cancellation is an important technique which allows the forward and reverse channels to overlap. This scheme is found in the CCITT V.32 recommendation for a full-duplex 9600 bps per second modem in which the modem simultaneously receives information over the same frequency band on which it is transmitting. The same scheme is found in the CCITT V.32 Bis recommendation. For a modem to simultaneously receive information over the same frequency band on which it is transmitting, however, each modem must be able to substantially cancel out the "echoes" of its own transmitted signal. Echoes are reflections of the transmit signal typically caused by a discontinuity in the transmission path of the signal, such as an impedance mismatch.
High speed modems for terrestrial applications often use echo cancellation techniques to make full use of the available bandwidth simultaneously in both directions. Echo cancellers are designed to cancel the portion of the received signal associated with the local transmitter. V.32 and V.32 Bis use a method called "echo cancellation" to eliminate the reflected transmitted signal. This method involves subtracting a locally synthesized replica of the reflected transmitted signal from the composite received signal. High speed modems utilizing the CCITT V.32 Bis standard are known to those of ordinary skill in the art and are commonly available, and such modems are made by several companies.
Echo cancellers are typically predictor structures that predict the incoming echo samples of a modem transmitter based on knowledge of the transmitter's past output samples. A typical modem echo canceller structure simply subtracts a weighted sum of some number of past outputs from the current receive sample. If the weighting function is properly chosen, and certain linearity conditions hold, the receive signal is essentially free of echo from the local transmitter. The recently developed Pulse Code Modulation (PCM) modems also take advantage of echo cancelling to achieve higher data rates.
In fact, PCM modems require a much deeper echo canceller null than previous generation modems. As a result, PCM modems have greater echo canceller sensitivity to cold-start modem impedance shifts. Divergence of the echo canceller in an echo cancelling PCM modem significantly effects transmission error rates due to the inability of the modem to properly discern the pulse code modulated levels in the presence of an echo signal. Moreover, slow changes in transmission medium characteristics over time such as that caused by temperature changes and the like can have a profound impact on modem error and transmission rates. Therefore it would be desirable to allow the PCM modem to quickly tune its echo canceller without a full retrain procedure.